Electric vehicles are powered by vehicle-mounted power supplies and drive wheels with motors. During use, they will not produce exhaust gas and pollute the environment. They have become an advantageous development model driven by carbon dioxide emission reduction policies, and are sought after by global vehicle manufacturers and users. Electric vehicles also have the advantages of low cost of use, high energy utilization, relatively mature technology and simple structure, but compared with traditional internal combustion engine vehicles, the serious problem of cruising range reduction caused by air conditioning has greatly reduced its competitive advantage.
In winter, the available waste heat of electric vehicles is limited, which cannot meet the needs of occupant heating and battery heating, and additional heaters need to be arranged. At present, there are usually two modes of electric vehicle heating in winter, various types of heat pumps and positive temperature coefficient resistance heating systems.
Zhang Hao and others analyzed the heating performance of the heat pump system and the positive temperature coefficient resistance heating system through experiments, and found that the heating power of the positive temperature coefficient resistance heating system is about 3kw at the ambient temperature of -5~3°C, which shows the energy consumption of the passenger compartment heating in winter The load is heavy.
Peng Qingfeng and others designed a new type of heat pump air conditioning system for electric vehicles, and compared it with the electric heating heating method in real vehicles. The results show that the heat pump system can save energy by more than 15%.
In addition to alleviating the serious attenuation of electric vehicle mileage in winter by developing a heat pump system, some scholars have conducted research on the compressor control strategy and refrigerant flow distribution of the heat pump system.
Zheng Linan proposed a low-pressure air-supplement heat pump air-conditioning system, and based on the KULI simulation software, studied its heating performance in winter, and established a corresponding compressor control strategy.
Xingxu et al proposed a new control strategy for the heat pump air conditioning system, and used the PID control principle to control the superheat of the branch. However, based on the existing heating schemes and control strategies, the heating of the passenger compartment of electric vehicles in winter still has a greater impact on the cruising range of the vehicle, and it is imminent to optimize heating energy consumption.
Considering that excessive humidity in the passenger compartment will cause fogging of the front windshield, and excessive carbon dioxide concentration will affect driving safety, the car basically adopts a full external circulation heating scheme under heating conditions. The heating of the passenger compartment consumes a lot of energy during external circulation. If the return air is properly introduced, the energy consumption of heating can be reduced to a certain extent. For this reason, various car companies and parts suppliers are working on corresponding technical problems. However, at present, there are few studies on the influence of return air, and the influence of air volume on heating rate and energy consumption is not considered. In order to support the development of air conditioning system control strategies for OEMs, this paper intends to conduct in-depth research on related issues.
In this paper, through the method of system simulation, the electric vehicle with Positive Temperature Coefficient (PTC) as the heating component is studied. Based on KULI software, a simulation model is built to analyze the influence of air volume and return air ratio on heating rate and heating energy consumption.
1 simulation model
The electric vehicle heating circuit using PTC as the heating component is shown in Figure 1. It is mainly composed of a water pump, PTC, heater core and blower, etc. Motor waste heat recovery and battery heating are not considered in this article.
MagnaKULI is a system-level thermal balance simulation software that is widely used in the field of vehicle thermal management simulation analysis. In this paper, KULI15.0 software is used to establish the simulation model. The simulation model is shown in Figure 2.
In order to reflect the real heating conditions to the maximum extent, a mass point is added in the simulation model as a heat capacity element to prevent sudden changes in water temperature caused by changes in PTC power.
The passenger cabin model used for heating is in the warmupsimpleCabin system shown in Figure 2. Its volume is 3.8m3, the outer surface area of the body is 18m2, and the area and inclination angle of the strong windshield are 1.2m2 and 32o respectively. In order to evaluate the maximum heating performance of the system, the heating analysis condition is cloudy, that is, the solar radiation is 0w/m2.
The PTC used in this system is a water heating heater with a maximum heating power of 7.5kw and a thermal efficiency of 0.9. In actual use, due to the limitation of water temperature and water flow, PTC often cannot work at the desired power, and the relationship between its maximum heating power and inlet water temperature and water flow is shown in Figure 3. The power of the water pump is 100w, and the blower has 7 gears. The heating air volume and power in different gears are shown in Table 1.
2 result analysis
2.1 The influence of stalls on heating performance
2.1.1 The influence of stalls on heating rate
In order to adapt to the differences in the population and the needs of use, there are multiple gears for the air outlet of the car air conditioner. In the initial stage of heating, users generally believe that the greater the air volume, the better the heating effect, and the temperature in the cabin will reach the target temperature faster. If the influence of wind speed on the thermal comfort of the occupants is ignored, and only the temperature rise rate of the occupant compartment is used as the evaluation index, the highest air-shielding volume is not necessarily the best heating solution due to the large load of the air-conditioning system during external circulation.
This paper analyzes the influence of different ambient temperatures (-20/-10/0/10/20°C) gears on the heating rate of the passenger compartment, and the results are shown in Figure 4. During the simulation process, the target air temperature at the foot of the passenger compartment is set to 25°C. It can be found from Figure 4 that the air volume corresponding to the highest heating rate is often not the maximum air volume, but is affected by the ambient temperature. When the ambient temperature is 20°C, the optimum heating air volume is 7 gears, and as the ambient temperature drops, the corresponding heating air volume gradually decreases. When the ambient temperature is -20°C, the air temperature target cannot be achieved in the passenger cabin under the 6th and 7th wind heating schemes.
Figure 5 summarizes the air volume gear corresponding to the ambient temperature and the fastest temperature rise. It can be seen that the ambient temperature has a great influence on the optimal heating air volume. As the ambient temperature changes from -20°C to 20°C, the optimal heating air volume changes from 4 Shift to 7th gear. This conclusion provides a reference for users to choose the best air volume in winter, and can also provide guidance for OEMs to formulate automatic air conditioning control strategies.
2.1.2 Effect of stalls on heating energy consumption
In the previous section, the influence of the air volume gear on the heating rate of the passenger compartment was analyzed. In addition, it is not clear whether the air volume has an impact on the cruising range of the vehicle. This section will carry out related analysis work. Assuming that the user’s single commute time is 60 minutes, the air volume is set to 4 after the average temperature of the foot air in the cabin reaches 25°C, and the heating energy consumption is obtained through simulation calculation. Heating energy consumption includes the energy consumption of PTC, blower and water pump, and its calculation formula is as follows:
In the formula: PT is power consumption (kk h): Pp is power consumption of water pump (kk h): PB is power consumption of blower (kk h): PH is power consumption of PTC (kk h).
The energy consumption results of a single commute under different schemes are shown in Table 2. Considering that the thermal comfort of the occupants is poor when the air volume is lower than the 4th gear, the heating scheme of the 1-3 air volume is abandoned. It can be seen from Table 2 that the air volume not only affects the heating rate, but also has a certain impact on the heating energy consumption. Therefore, the influence of the air volume should be considered when formulating the control strategy of the air conditioning system.
2.2 Influence of return air on heating performance
2.2.1 Effect of return air on heating rate
In winter, the temperature of the front windshield of the car is low. If the humidity in the passenger compartment is too high, the windshield is prone to fogging, which will affect driving safety. Therefore, the car basically adopts a full external circulation heating scheme under heating conditions. The external circulation has a great impact on the energy consumption brought by the heating of the passenger compartment. If the return air is properly introduced, the heating energy consumption can be reduced to a certain extent. For this reason, various car companies and parts suppliers are tackling corresponding technical problems. However, the effect of the return air ratio on reducing the heating energy consumption of the passenger compartment is not clear, so this paper carried out relevant analysis work.
According to different ambient temperatures (-20/-10/0/10/20°C), the heating rate of the passenger compartment was analyzed when the return air ratio was 0%, 10%, 20% and 30%, respectively, and the results are shown in Figure 6 . It can be seen from the figure that as the return air ratio increases, the time required for the passenger compartment to reach the target temperature is shortened, which means better heating effect, and the lower the ambient temperature, the more obvious the improvement. When the ambient temperature is -20°C, if the return air ratio is 0%, the 6th and 7th wind cannot meet the heating requirements; when the return air ratio is 20%, the 6th wind can meet the heating demand; when the return air ratio is 30% ,7 The windshield can meet the heating demand. Thermal comfort is not only related to air temperature, but also to wind speed. Some large sUVs have a large space, and if the air volume is small, the thermal comfort of the occupants will be affected. At this time, if the return air is properly introduced, the upper limit of the maximum air volume can be increased, thereby better improving the overall thermal comfort.
2.2.2 The influence of return air on the cruising range of the vehicle
The introduction of return air can not only increase the heating rate of the passenger compartment, but also reduce heating energy consumption. In order to evaluate the impact of different return air ratios on heating energy consumption, this section analyzes an electric vehicle. The total power of the power battery of an electric vehicle is 90kw h, and the driving energy consumption under one CLTC (China Light Vehicle Test Cycle) cycle is 4.06kw h, and the pure electric cruising range is 643km without the air conditioner. Assuming that the user uses the car for 60 minutes each time in winter, under different heating schemes, first select the air volume of the fastest heating gear, and choose 4 windshields after the temperature in the cabin is stable. The energy consumption of the vehicle includes the energy consumption of the drive and the energy consumption of the air conditioning system. After comprehensive calculation, the cruising range of the vehicle under different heating schemes is shown in Figure 7.
It can be seen from Figure 7 that the winter air-conditioning system has a great impact on the cruising range of the vehicle, and the cruising range at an ambient temperature of -20°C has dropped by more than 50% compared with normal temperature. The mileage effect in winter is remarkable. Horizontal comparison can also be found that with the increase of ambient temperature, the effect of the return air on the cruising range is gradually weakened. The 30% return air at an ambient temperature of -20°C is 27km higher than the cruising range without return air, while the 20°C environment The 30% return air at low temperature is only 8km longer than the cruising range without return air.
This paper takes an electric vehicle with PTC as the heating scheme as the analysis object, and analyzes the influence of return air ratio and air volume on the heating effect of electric vehicles under different ambient temperatures. The main research conclusions are as follows:
(1) The air volume corresponding to the highest heating rate is often not the maximum air volume, but is related to the ambient temperature. When the ambient temperature is 20°C, the optimal heating air volume is 7 gears, as the ambient temperature drops, the corresponding heating air volume gradually decreases, and when the ambient temperature is -20 °C, the optimal heating air volume is 4 gears.
(2) The air volume not only affects the heating rate, but also has a certain impact on the heating energy consumption. Therefore, the influence of the air volume should be considered when formulating the control strategy of the air conditioning system.
(3) With the increase of the return air ratio, the time required for the passenger compartment to reach the target temperature is shortened, and the maximum working air volume under low temperature conditions can be increased.
(4) At -20°C ambient temperature, 30% return air improves the cruising range by 27km compared with no return air. % return air only increased by 8km.